In order to enhance the best efficiency performance, solar cell plants have gradually introduced local back surface field (LBSF) technology since 2013. The operating mode of this technique is Passivated Emitter and Rear Contact (PERC). The process uses Atomic Layer Deposition (ALD) or Chemical Vapor Deposition (CVD) to deposit SiOx, TiOx, or AlOx on the silicon wafer to serve as the back passivation layer, and then deposit SiNx by CVD process to form a capping layer. The main function of the passivation layer in the local back surface field solar cell is to repair defects on the surface of the silicon wafer. Because the cutting and processing of the silicon wafer will produce amorphous silicon having more dangling bonds, which, at the edge of the silicon wafer, will recombine the carriers generated after the silicon wafer is exposed to light, thereby reducing the carrier life time and deteriorating the electrical properties. However, the local back surface field solar cell will have increased open circuit voltage (Voc) and short circuit current (Isc) due to the function of the passivation layer, and thus can obtain significantly increased photoelectric conversion efficiency. Nevertheless, the passivation layer provides only limited aluminum-silicon contact, and therefore the series resistance (Rs) will be increased and the fill factor (FF) will be reduced.
The development of PERC aluminum paste is derived from silicon-based solar cells with a back passivation layer. The main difference between local back surface field solar cells and conventional solar cells is that the aluminum paste used in conventional silicon-based solar cells is screen printed on the back surface of the silicon wafer, in which the aluminum layer directly contacts the silicon wafer and forms a whole back surface field (BSF) after sintering, and, however, in the local back surface field solar cells, most (>95%) of the aluminum layer covers the SiNx capping layer, leaving only a limited laser opening area to directly contact aluminum with silicon, which further forms a local back surface field (LBSF) after sintering.
One of the major technical problems faced by LBSF technology is that if the aluminum layer attaches to the SiNx capping layer, then the passivation layer may be damaged. If the passivation layer is damaged, the local back surface field solar cell becomes unable to maintain improved open circuit voltage (Voc), short-circuit current (Isc), electrical properties, and conversion efficiency. Secondly, because the contact between the aluminum layer and the silicon wafer only passes through the limited area of the laser opening, under the influence of appropriate amount of glass powder (1.0-5.0 wt %, based on the total weight of the aluminum paste), the particle size of the aluminum powder itself in the aluminum paste formulation and the thickness of the alumina layer on the surface of the aluminum powder will become critical to the occurrence of powder extraction, aluminum beads, and voids at the LBSF positions that affect the quality of the local back surface field solar cell during the aluminum-silicon eutectic melting process.
However, for today's silicon-based solar cells with the back passivation layer, all focus is on the application of glass frit to the aluminum paste formulation for controlling the erosion and adhesion capability of the aluminum layer to the back passivation layer (SiOx, TiOx, or AlOx/SiNx) of the local back surface field solar cell. It can be seen that all the prior arts ignore the influence of the aluminum powder, which accounts for more than 60 wt % of the aluminum paste, on the whole quality characteristics and reliability of the local back surface field solar cell during the sintering process, and the above technical problems have never been addressed or advised, either. For example, there has been proposed an aluminum paste containing 60 to 87 wt % of aluminum powder and glass frit having a high lead content. The adhesion between the aluminum paste and the capping layer is strengthened by utilizing the easily meltable and easily reactive properties of the lead oxide (PbO). However, in the prior arts, the influence of the issues, such as the powder extraction, aluminum beads, the adhesion of the aluminum layer to the SiNx capping layer, the voids occurred at the LBSF positions, etc. incurred by the aluminum paste containing more than 60 wt % aluminum powder during the sintering of the aluminum paste with silicon, on the whole quality characteristics and reliability of the local back surface field solar cell has never been investigated.
Therefore, how to develop an aluminum paste that can be used in LBSF and eliminate the issue of the photoelectricity conversion efficiency of the local back surface field solar cell incurred by the occurrence of phenomena, such as the powder extraction of aluminum layer, the aluminum beads, the adhesion of the aluminum layer to the SiNx capping layer, the voids at the LBSF positions, etc., is now the technical focus of all LBSF research and development personnel.